This invention relates to fluid delivery systems which include centrifugal pumps and, more particularly, to an integrated pumping system comprising multiple pumping elements which are adaptable over a large range of fluid flow rates.
Many fluid delivery systems require a very large flow range with high pump head rise and low pump temperature rise throughout the flow range. For example, current afterburning gas turbojet engines require significantly higher fuel delivery rates while operating in the augmented (or afterburning) mode than when operating in the non-augmented mode. The initial problem facing fuel delivery system designers, therefore, was to provide a single pump which would operate over the entire flow range with minimum fuel temperature rise from the delivery system. The minimum fuel temperature rise requirement was easily attained at high flow rates because of the short dwell time in the system. However, the low temperature rise requirement at low pump flow rates was more difficult to attain. A centrifugal pump having a variable diffuser which utilizes a slotted shutter valve to selectively close off diffuser vane entry passages, as the fuel rate of the pump is decreased, has been shown to substantially improve the desired flow range and low speed temperature rise characteristics. Such a pump is described and claimed in U.S. Pat. No. 3,784,318, by the present inventor and assigned to the assignee of the present invention. A simple means for controlling the position of the shutter valve is described and claimed in U.S. Pat. No. 3,826,586, J. C. Richards, and is also assigned to the assignee of the present invention.
While the shuttered diffuser centrifugal pump has proven successful, it has been unable to provide sufficient pressure rise at low pump speeds such as encountered while starting a gas turbine engine. Since pressure rise of a centrifugal pump is essentially proportional to the pump speed squared, it becomes apparent that the pressure rise of a pump operating in the starting speed range (10% to 40% of design speed) is relatively insignificant. Accordingly, it is the custom to incorporate a displacement type of pump (vane, gear, piston, etc.) for use in the starting and unaugmented engine operating modes, and a centrifugal pump for use in supplying the high augmentation flow rates, since the displacement pump is capable of providing adequate fuel pump pressure at low engine speeds. Furthermore, efficient centrifugal pumps operating at high rotational speeds require preboosting of the working fluid to prevent cavitation within the pump.
Attempts to unitize the fluid delivery system by combining a displacement type starting and low flow rate element, an inlet boost element and a centrifugal (or displacement) high flow element have generally resulted in complicated, large, heavy packages which are undesirable for aircraft engine applications.